Forms of radiation therapy for cancer that provide highly precise treatments are being introduced. Such forms of radiation therapy include stereotactic radiation therapy (SRT) in which pinpoint radiation therapy is performed, and intensity modulated particle therapy (IMPT) in which the radiation field can be set three-dimensionally along the contour of cancer, while changing the dose intensity within the same radiation field. In these therapeutic methods, the integral value (i.e., dose distribution) of amounts of microscopic energy deposited in three-dimensional positions of the target is precisely adjusted. Furthermore, particle therapy, which uses charged particle beams with high dose concentration, such as proton beams or heavy ion beams (e.g., carbon and neon beams), has been practiced. Particle therapy has the advantage of treating tumors by controlling the irradiated position and radiation dose with higher precision than conventional X-ray therapy. Particle therapy is required to emit energy properly from a particle beam into the target position such as a lesion in an in vivo tissue, and simultaneously minimize the effect on normal tissue surrounding the target. For these purposes, radial spread of the particle beam and the position of the Bragg peak of the particle beam are aligned with the target position in the object to be irradiated.
In actual radiation therapy treatment planning, the dose distribution in three-dimensional positions within an in vivo tissue is optimized. In typical treatment planning, the dose distribution (radiation dose in each position) within the target tissue is modified in accordance with the therapeutic purpose, and simultaneously, the effect of radiation on the surrounding normal tissue is minimized, resulting in a minimal effect on an organ at risk. To create a dose distribution with such a complicated shape, beams may be precisely controlled, and may be emitted from multiple directions. To control in this way, a filter, a collimator, and the like (a range shifter, a multileaf collimator, a bolus, and the like) adjusted to the object to be irradiated are equipped. To achieve such highly controlled radiation therapy, a high degree of quality assurance and quality control (hereinafter abbreviated to “QA/QC”) is required for the entire apparatus including a radiation device, auxiliary instruments, a filter, a collimator, and the like, as well as the irradiation process using these devices.
The QA/QC for the treatment planning and the various devices requires a technology that enables actual measurement of a correct integral of amounts of energy deposited by multiple beams of ionizing radiation incident from various direction at various acceleration energies. The reason for this is that, if the dose in each position can be precisely measured by integrating the amounts of deposited energy, a three-dimensional distribution of deposited energy (dose distribution), which supports the QA/QC, can be measured. For this purpose, one- or two-dimensional dosimeters such as ionization chamber dosimeters, semiconductor detectors, or film-type dosimeters have been conventionally used. These dosimeters perform actual measurement of the above-described dose distribution along one- or two-dimensional coordinates, in a region where the particle beam is to be aligned to the target position. In addition to these dosimeters, gel dosimeters have recently attracted attention which can measure a three-dimensional dose distribution by means of a gel using the measurement principle of a chemical dosimeter. The use of a gel dosimeter provides an additional advantage in that amounts of energy deposited by radiation beams in the positions of water, which is a substance that can be regarded as equivalent to a living body, can be accurately measured, i.e., the effect of radiation on a living body-equivalent substance or a water-equivalent substance can be measured. A gel dosimeter can be used to obtain a three-dimensional dose distribution, while using the gel dosimeter per se as a solid phantom.
Fricke gel dosimeters (Patent Document 1) or polymer gel dosimeters (Patent Documents 2 to 4), for example, have been reported as gel dosimeters that can measure a three-dimensional dose distribution. A Fricke gel dosimeter is a gel containing a solution (aqueous solution containing ferrous sulfate) for a Fricke dosimeter known as a liquid chemical dosimeter. The Fricke gel dosimeter uses the phenomenon in which the oxidation reaction (coloration) of iron from the divalent to trivalent state due to radiation increases proportionately with the absorbed dose. On the other hand, a polymer gel dosimeter contains monomers dispersed in a gel. Upon irradiation, a polymer is produced proportionately with the dose, and thus, the dose can be estimated by determining the amount of the polymer produced (opacity). The produced polymer is characterized by being unlikely to diffuse in the gel, showing stable opacity over time, and being visually excellent in that the opaque portion appears floating in the transparent gel.